Biology
What Is Living?: Quick Revision
A brief note on What Is Living?: growth, reproduction, metabolism, cells and the response to the surroundings, which of them define life, and the meaning of a living organism. With a worked example, practice, a program and questions to check yourself.
Written by StemPy LabA brief note on What Is Living?. Try each question at the end before you open its answer.
Five features, three that define
| Feature | In every living thing? | Exception |
|---|---|---|
| Growth (more mass and cells, from inside) | no | an adult human, a dry seed |
| Reproduction (new individuals like the parents) | no | a mule, a worker bee |
| Metabolism (all the chemical reactions in the body) | yes | none |
| Cellular organisation (a body of cells) | yes | none |
| Response to the surroundings (consciousness) | yes | none |
- Non-living things such as a crystal grow only by material added on the outside.
- In a single-celled organism, growth and reproduction are one event.
- Asexual ways: binary fission (Amoeba, bacteria), budding (yeast, Hydra), fragmentation (filamentous algae, fungi), spores (fungi), regeneration (Planaria).
- A stimulus is physical (light, touch), chemical (a smell) or biological (another organism). Photoperiod affects seasonal breeders.
- Only human beings are known to be self-conscious.
- A living organism is a self-replicating, evolving and self-regulating interactive system capable of responding to external stimuli.
A worked example
Fire grows, spreads to new places and flickers in the wind. Is it alive?
- Growth. It gets bigger only by taking in fuel from outside, not by cells dividing.
- Spreading. It makes no copy of itself; it simply reaches new fuel.
- The defining features. It has one reaction, burning, but no cells and no body of thousands of reactions. Its flicker is not a sensed response.
Answer: fire is not alive. It lacks cells and metabolism, two defining features.
Practice with new numbers
A program to try
The program checks five things by the rule of the three defining features.
Mistakes to avoid
- Using one feature alone. A thermostat responds and a crystal grows, yet neither is alive.
- Calling a sterile animal non-living. A mule has metabolism, cells and a response.
- Thinking plants do not respond. They respond to light, water, touch and temperature.
- Calling a reaction in a test tube a living thing. It is a living reaction, not an organism.
Check yourself
- Fire spreads and grows. Name two defining features of life that it lacks.
- A filament of Spirogyra breaks into pieces, and each piece grows into a new filament. What is this way of reproducing called?
- What is the difference between consciousness and self-consciousness?
- A coral on a reef looks like a rock. How would you show that it is alive?
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